Internal combustion engines typically include an EGR system to recirculate a controlled portion of the engine exhaust gas into an engine intake manifold in order to provide a reduction in NO.sub.x emissions. Typically, a control mechanism including an EGR valve is provided to vary the EGR rate according to one or more sensed condition, such as engine temperature, air charge entering the intake manifold, and engine speed.
It is desirable to monitor the operation of the EGR system through onboard diagnostic routines to determine whether or not the system is operating as expected. One approach to EGR monitoring terminates the recirculation of exhaust gas by closing the EGR valve during steady state cruise operation and records the change or difference in manifold absolute pressure (MAP) between the value prior to termination and the value a short time period following termination. The waiting period allows the MAP signal from a MAP sensor to settle. This termination of recirculation is usually commanded several times and the MAP difference signal is averaged. A problem with this approach is that under high load, the manifold pressure does not change significantly between the value sensed with EGR ON (EGR valve open) and the value sensed with EGR OFF (EGR valve closed). Relying on the MAP difference value at high load may cause a false indication of a malfunctioning EGR system.